I have similar problems here:
running glxgears works mostly for 15 seconds. then the xserver freezes or the screens go blank. 99%cpu time then for the xserver-process. Sometimes i can kill it from another machine, sometimes not. its really annoying.
another problem with xawtv. there it freezes sometimes when i resize the tv window - or when xawtv runs and i try to start any other application (mostly firefox)

- flashed motherboard bios.
- compiled the driver with forced AGP down to 2 or 1.
The compiling results in a disabling of the driver, because it seems that my board doesnt support AGP Rates beyond 4. quite strange.

I have similar problems here:
running glxgears works mostly for 15 seconds. then the xserver freezes or the screens go blank. 99%cpu time then for the xserver-process. Sometimes i can kill it from another machine, sometimes not. its really annoying.
another problem with xawtv. there it freezes sometimes when i resize the tv window - or when xawtv runs and i try to start any other application (mostly firefox)

I went back to the old drivers and experienced the same crash issue, especially with Doom 3, which crashes every single time once the you begin to actually enter the game... after choosing your dificulty and it loads all the resources.

That lead me to believe that perhaps it was sound related, because while the screen is still blank, the sound starts and stutters, and then the computer crashes. This happens sometimes with other OpenGL games too.

I changed from ALSA drivers to OSS drivers, and Doom 3 didn't recognize the audio output and disabled the sound, and then the game loaded and ran fine... just without the sound. So, it would appear that at least on my system, there's some conflict between ALSA drivers (or perhaps when a program tries to use ALSA's OSS compatability drivers?) and OpenGL that causes the whole system to crash.

I went back to the old drivers and experienced the same crash issue, especially with Doom 3, which crashes every single time once the you begin to actually enter the game... after choosing your dificulty and it loads all the resources.

That lead me to believe that perhaps it was sound related, because while the screen is still blank, the sound starts and stutters, and then the computer crashes. This happens sometimes with other OpenGL games too.

The sound begins to shutter because, presumably, (at least by my experience) your nVidia driver crashed and the X process is eating 100% CPU time.
Try connecting to the downed machine using ssh after it had crashed and report back with your findings.